Oracle - Join most recent status at the time of activity - sql

Question

I have two tables - one with employee activity and one with employee_status. The issue is the employee status changes over time, so I need to join the status as it was at the time of the session.
>>> employee_activity
id session_start
emp1 1/1/2019
emp1 2/22/2019
emp1 3/1/2019
emp2 1/4/2019
emp2 2/23/2019
>>> employee_status
id status effective date
emp1 a 1/1/2018
emp1 b 2/1/2019
emp1 c 3/5/2019
emp2 a 6/1/2018
emp2 b 1/1/2019
So I started writing something that will make sure it's ignoring statuses after the activity, but I'm struggling a bit with figuring out how to only select the most recent status. The query needs join only the status with the max
effective date that is less than the session start
SELECT * FROM employee_activity a
LEFT join employee_status s on a.id = s.id WHERE s.effective_date <= a.session_start
-- how do I join only the most recent status?
The desired output from the two tables above would be
>>> my_output
id session_start status
emp1 1/1/2019 a
emp1 2/22/2019 b
emp1 3/1/2019 b
emp2 1/4/2019 b
emp2 2/23/2019 b
Thanks!!

Answer 1

Calculate first the validity interval from the STATUS, i.e. instead of EFFECTIVE_DATE you have starting and ending timestamp.
Note, that I use a default open end date and I subtract one second from the end date to get closed interval which can be queried using BETWEEN.
Than simple join on the key and add the between constraint for the time:
with emp as (
select ID, STATUS, EFFECTIVE_DATE status_valid_from,
lead(EFFECTIVE_DATE - INTERVAL '1' SECOND,1,DATE'2500-01-01')
over (partition by id order by EFFECTIVE_DATE) as status_valid_to
from employee_status)
SELECT a.id, a.SESSION_START, s.STATUS, s.STATUS_VALID_FROM
FROM employee_activity a
LEFT join emp s
on a.id = s.id and session_start between s.status_valid_from and s.status_valid_to
order by 1,2;
ID SESSION_START S STATUS_VALID_FROM
---- ------------------- - -------------------
emp1 01.01.2019 00:00:00 a 01.01.2018 00:00:00
emp1 22.02.2019 00:00:00 b 01.02.2019 00:00:00
emp1 01.03.2019 00:00:00 b 01.02.2019 00:00:00
emp2 04.01.2019 00:00:00 b 01.01.2019 00:00:00
emp2 23.02.2019 00:00:00 b 01.01.2019 00:00:00
Sample Data
create table employee_activity as
select 'emp1' id, to_date('1/1/2019','mm/dd/yyyy') session_start from dual union all
select 'emp1' id, to_date('2/22/2019','mm/dd/yyyy') session_start from dual union all
select 'emp1' id, to_date('3/1/2019','mm/dd/yyyy') session_start from dual union all
select 'emp2' id, to_date('1/4/2019','mm/dd/yyyy') session_start from dual union all
select 'emp2' id, to_date('2/23/2019','mm/dd/yyyy') session_start from dual;
create table employee_status as
select 'emp1' id, 'a'status, to_date('1/1/2018','mm/dd/yyyy') effective_date from dual union all
select 'emp1' id, 'b'status, to_date('2/1/2019','mm/dd/yyyy') effective_date from dual union all
select 'emp1' id, 'c'status, to_date('3/5/2019','mm/dd/yyyy') effective_date from dual union all
select 'emp2' id, 'a'status, to_date('6/1/2018','mm/dd/yyyy') effective_date from dual union all
select 'emp2' id, 'b'status, to_date('1/1/2019','mm/dd/yyyy') effective_date from dual;

Answer 2

You can do this using a correlated subquery:
select ea.*,
(select max(es.status) keep (dense_rank first order by es.effective_date desc)
from employee_status es
where es.id = ea.id and es.effective_date <= ea.session_start
) as status
from employee_activity ea;
In Oracle 12C+, there is the more intuitive:
select ea.*,
(select es.status
from employee_status es
where es.id = ea.id and es.effective_date <= ea.session_start
order by es.effective_date desc
fetch first 1 row only
) as status
from employee_activity ea;